A plain bearing is a bearing in which friction is reduced through low resistant sliding between a surface of the bearing and a surface of a sliding body brought into contact with the surface of the bearing. A lubricant is often used in order to achieve a reduction in friction, and an oil, grease, or the like is used by being applied to a sliding interface. However, the oil or grease to be applied causes, for example, scattering contamination, such as dripping or leaking of a liquid, or an increase in friction coefficient resulting from a shortage of the liquid. Therefore, it is difficult to obtain a stable friction coefficient for a long period of time. Under such circumstances, a material having a self-lubricating property not requiring the oil or grease has attracted attention. A self-lubricating material is a material exhibiting lubricating characteristics by itself without the need for external addition. Typical examples thereof include, as resin materials, polyacetal (POM), polyethylene (PE), and polytetrafluoroethylene (PTFE). Those resin materials are molded and used as the bearing. In addition, the self-lubricating material is often used in combination with the lubricant through addition of the oil or grease.
PV characteristics are often used as a reference for selecting many existing bearing materials. This is a value represented by a product of a sliding speed V and a load P, which serve as the use conditions of a bearing, and it is necessary to use the bearing within its allowable PV value specific to its material. In addition, the bearing also has an allowable speed and an allowable load, which are each known as a parameter required for selection of the material of the bearing as a value specific to the material. In use under the conditions of high speed and high load in consideration of the allowable speed and allowable load, an ultrahigh molecular resin material, for example, polyether ether ketone (PEEK) or the like is used as a hard material. However, a material having a wide allowable PV value range has a high friction coefficient, and there is a limit to reducing friction. On the other hand, as a material having a low friction coefficient, there is given PTFE. However, PTFE has a narrow allowable PV value range and suffers a large amount of wear when used under high load, with the result that stable friction is not obtained. Such wear is more remarkable in use in an oilless state, and hence an oil is generally used in combination.
In addition, an additive material has been mixed with the material having a low friction coefficient to increase its hardness and widen its allowable PV value range. In this case, a glass filler, a fiber, or the like is used as the additive material. However, at present, such additive material causes an increase in friction coefficient and low friction cannot be maintained. In view of the foregoing, in PTL 1, an investigation has been made on film formation and molding of a resin material on a surface of a metal member.
The technology of PTL 1 provides an improving effect on the allowable load by virtue of an influence of the metal member, but a sliding surface is formed of the original resin material and hence improvement in wear resistance is insufficient. It is hard to say that the allowable speed is sufficiently improved.
As described above, the material having a wide allowable PV value range has a high friction coefficient, and the material having a low friction coefficient has a narrow allowable PV value range. Therefore, there is a demand for a configuration or material having a wide allowable PV value range and a low friction coefficient. Further, in recent years, the demand for high functionalization, compactification, and energy saving has led to severer conditions of the sliding speed V and the load P. However, at present, no plain bearing material has a PV value with which the bearing can respond at high speed under high load in an oilless state.